The invention relates to the field of electronic reproduction technology and concerns a method for engraving printing cylinders using an engraving member in an electronic engraving machine for producing printing forms for gravure printing.
In an electronic engraving machine, an engraving member having a stylus as a cutting tool moves along a rotating printing cylinder in the axial direction. The stylus, which is controlled by an engraving signal, cuts a series of depressions, called steps, into the surface of the printing cylinder, which are arranged in a grid. The engraving signal is formed by superimposing an image signal representing the tone values between xe2x80x9clightxe2x80x9d and xe2x80x9cdarkxe2x80x9d with a periodic grid signal. While the grid signal effectuates a vibrating stroke motion of the stylus for generating the grid, the image signal values corresponding to the tone values that are to be reproduced determine the geometric parameters such as the transverse diagonals and the longitudinal diagonals of the engraved steps.
The engraving member comprises an electromagnetic drive system for the engraving stylus, for example. The electromagnetic drive system essentially is formed of a stationary electromagnet which is charged with the engraving signal and a rotation system whose armature moves in the air gap of the electromagnet. The rotation system comprises the armature, an armature axle, an axle bearing, and a damping mechanism. One end of the armature axle is designed as a torsion bar that has been clamped in place permanently, while the other end carries a lever type stylus holder for the engraving stylus. The engraving member also comprises a mechanical slide foot and a ductor. The slide foot, which braces on the printing cylinder during engraving, provides for a defined spacing between the engraving member and the surface of the printing cylinder. The ductor removes the material that emerges during the cutting of the steps from the printing cylinder.
A magnetic electrical alternating field is generated in the electromagnet by the engraving signal. The alternating field exerts an alternating electrical torque on the armature, which is counteracted by the torque of the torsion bar. The alternating electrical torque guides the shaft out of the neutral position, which is defined by the torsion bar, by an angle of rotation that is proportional to the respective engraving signal value. With of the rotation of the shaft, the engraving stylus executes a stroke motion that is directed to the surface of the printing cylinder, which motion defines the depth of stylus penetration into the printing cylinder, and thus the geometric parameters of the engraved steps.
The magnetic alternating field in the electromagnet generates eddy current losses in the armature and in the yoke of the electromagnet, which heat the armature axle and the stylus holder with the stylus. Additional heating of the engraving member is caused by hysteresis losses in the yoke and in the armature, losses due to mechanical damping of the rotation system, and ohmic losses due to the image signal as a portion of the engraving signal.
A change in the operating temperature of the engraving member affects the depth of penetration of the stylus into the printing cylinder, and as a result incorrect tone values are engraved.
DE-A-197 22 996 teaches a method for signal processing in an electronic engraving machine in which an image signal which represents the tone values that are to be engraved and a grid signal for generating a printing grid are converted in accordance with a transfer function that is defined by transmission coefficients into an engraving signal for controlling an engraving member.
EP-A-0 595 324 teaches a method for automatically calibrating the engraving signal in an electronic engraving machine in which the engraved actual tone values are measured out and compared to predetermined desired tone values, and settings are derived from the comparison, with which the engraving signal is calibrated such that the actual tone values correspond to the desired tone values. Neither reference teaches measures for compensating the disturbing influences of temperature fluctuations in an engraving member on the tone values of the engraved steps.
It is thus the object of the present invention to improve a method for engraving printing cylinders using an engraving member in an electronic engraving machine such that disruptive influences of temperature changes in the engraving member on the engraving tone values are compensated in order to achieve a good engraving quality.
According to the method of the present invention for engraving printing cylinders using an engraving member and an electronic engraving machine, with an engraving stylus controlled by an engraving signal of an engraving member, engraving the series of steps arranged in a printing grid into a rotating printing cylinder. An image signal that represents tone values between xe2x80x9cwhitexe2x80x9d and xe2x80x9cdarkxe2x80x9d of the steps that are to be engraved and a periodic grid signal for generating the printing grid are converted into the engraving signal in accordance with the transfer function. To compensate disturbing influences of operating temperature fluctuations of the engraving member on the tone values of the steps, operating temperature in the engraving member is measured at at least one measurement location. The engraving signal is corrected for compensating fluctuations of the operating temperature in that at least one transmission coefficient of the transfer function is modified dependent on at least one measured operating temperature.
In another embodiment of the invention, the image signal is filtered prior to being superimposed with the grid signal with the filtered signal being used as the engraving signal. An operating temperature in the engraving member is measured at at least one measurement location. For compensating fluctuations and operating temperature, the engraving signal is corrected by modifying a filter function dependent on at least one measured operating temperature.
In a further embodiment of the invention, an operating temperature in the engraving member is measured at at least one measurement location, and at least one of temperature of at least one component of the engraving member and of air surrounding the engraving member is modified dependent on at least one measured operating temperature.